Water-absorbing resins are widely used in sanitary goods, hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.
Such water-absorbing resins are termed “superabsorbent polymers,” or SAPs, and typically are lightly crosslinked hydrophilic polymers. SAPs generally are discussed in Goldman et al. U.S. Pat. Nos. 5,669,894 and 5,559,335, each incorporated herein by reference. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirement for an SAP used in a hygienic article, such as a diaper.
As used here and hereafter, the term “SAP particles” refers to superabsorbent polymer particles in the dry state, i.e., particles containing from no water up to an amount of water less than the weight of the particles. The terms “SAP gel” or “SAP hydrogel” refer to a superabsorbent polymer in the hydrated state, i.e., particles that have absorbed at least their weight in water, and typically several times their weight in water.
The development of highly absorbent, SAP-containing articles for use as disposable diapers, adult incontinence pads and briefs, and catamenial products, such as sanitary napkins, is the subject of substantial commercial interest. A highly desired feature of such absorbent articles is thinness. For example, thin diapers are less bulky to wear, fit better under clothing, and are less noticeable. Article packaging also is more compact, which makes the diapers easier for the consumer to carry and store. Packaging compactness also results in reduced distribution costs for the manufacturer and distributor, including less required shelf space per diaper unit.
A variety of parameters effect the ability of an SAP particle and an absorbent core to rapidly absorb a large amount of an aqueous fluid, and then retain the absorbed fluid under various stresses. Optimization of these parameters allows a reduction in amounts of cellulosic fiber, or fluff, present in a diaper core, which in turn reduces the overall bulk of the diaper. SAP particles and cores, therefore, are designed in an attempt to optimize absorption capacity, absorption rate, acquisition time, gel strength, and permeability, and decrease core thickness.
The present invention is directed to the surprising and unexpected finding that an absorbent sheet material prepared from an acidic water-absorbing resin, a basic water-absorbing resin, and plasticizer, after thermal pressing, provides a flexible sheet of excellent structural integrity. The sheet exhibits excellent fluid absorption and retention properties, and allows the elimination or reduction of the amount of fluff in the absorbent sheet material.
Absorbent article cores typically contain a relatively low amount (e.g., less than about 50% by weight) of SAP particles for several reasons. First, SAPs employed in present-day absorbent articles lack an absorption rate that allows the SAP particles to quickly absorb body fluids, especially in “gush” situations. This necessitates the inclusion of fibers, typically wood pulp fibers, admixed with an SAP in the absorptive core of the article as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.
In order to manufacture a sheet material for a diaper core substantially, or completely, free of cellulosic fiber, a continuous zone of SAP particles is required. However, because of the nature of SAP particles, it has been impossible to combine features such as high absorption capacity and high gel strength in an SAP because improving one feature adversely affects the other.
In particular, SAPs typically exhibit gel blocking. “Gel blocking” occurs when SAP particles are wetted, and the SAP particles, swell to retard fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member takes place via a slow diffusion process. Gel blocking can be a particularly acute problem if the SAP particles do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. In practical terms, the acquisition of fluids by the absorbent article is much slower than the rate at which a fluid is discharged, especially in a gush situation. Leakage from and/or deformation of the absorbent article can take place well before the SAP particles in the absorbent article are fully saturated or before the fluid can diffuse, or wick, through the “blocking” particles into the remainder of the absorbent core.
The gel blocking phenomena has necessitated the use of a fibrous matrix to disperse the SAP particles, and separate the SAP particles from one another. The fibrous matrix also provides a capillary structure that allows fluid to reach SAP located in regions of the core remote from the initial fluid discharge point. However, dispersing a relatively low amount of the SAP in a fibrous matrix to minimize or avoid gel blocking reduces the total fluid storage capacity of absorbent cores. Overall, using lower amounts of an SAP limits the advantage of the SAP, i.e., an ability to absorb and retain large quantities of body fluids per given volume.
For absorbent sheet materials containing a relatively high amount of SAP particles, other SAP properties also are important. It has been found that the openness, or porosity, of the hydrogel layer formed when the SAP swells in the presence of body fluids helps determine the ability of an SAP to acquire and transport a fluid, especially when the SAP is present in high amounts in the absorbent core. Porosity refers to the fractional volume of a particle that is not occupied by solid material. For a hydrogel layer formed entirely from an SAP, porosity is the fractional volume of the layer that is not occupied by hydrogel. For an absorbent structure containing the hydrogel, as well as other components, porosity is the fractional volume (also referred to as void volume) that is not occupied by the hydrogel or other solid components (e.g., cellulosic fibers).
U.S. Pat. Nos. 4,076,673 and 4,861,539 disclose an absorbent article containing a superabsorbent film prepared by extruding a solution of a linear absorbent polymer and subsequently crosslinking the polymer. The resulting SAP film can absorb significant quantities of liquids, but has limited liquid transport properties because the film is essentially nonporous, i.e., lacks internal capillary channels. Such an SAP film, therefore, is especially prone to gel blocking. Furthermore, because the crosslinking reaction between the hydroxy groups of the crosslinking agent, e.g., glycerol, and the carboxy groups of the SAP is relatively slow, i.e., glycerol-treated linear polymers typically are cured at 200° C. for 50 minutes, relatively brittle sheets of bonded absorbent particles result. These friable sheets are difficult to handle, especially in the manufacture of the ultimately desired absorbent articles.
Absorbent sheets of the present invention overcome problems associated with diaper cores containing absorbent sheets having a high percentage of SAP. A sheet material of the present invention is flexible and has high structural integrity for ease of sheet manufacture, handling, and the manufacture of diaper cores. A present sheet material is prepared from a mixture of an unneutralized acidic water-absorbing resin, an unneutralized basic water-absorbing resin, and a plasticizer, which, after thermal pressing, provides an improved absorbent sheet material for use in an absorbent article. The acidic and basic water-absorbing resin can be present as a physical blend, i.e., a mixed bed of acidic resin particles and basic resin particles, or the acidic and basic resins can be present in a single multicomponent superabsorbent resin particle.
In addition to providing an absorbent sheet material having excellent fluid absorption and retention properties, the sheet material is sufficiently flexible and has sufficient structural integrity to be continuously manufactured and formed into rolls for economical transport and storage. Rolls of the sheet material facilitate the manufacture of absorbent articles, especially absorbent articles that are free of fluff.
Other investigators attempted to manufacture a continuous roll of a sheet material that contains a high percentage of a particulate SAP, such as sodium polyacrylate. Examples of fibrous substrates impregnated with superabsorbent polymer are found in U.S. Pat. Nos. 5,614,269; 5,980,996; and 5,756,159, wherein a fibrous substrate is impregnated with the monomer, which subsequently is polymerized in situ by UV light to form an SAP in contact with the fibrous substrate.
Other patents, including U.S. Pat. Nos. 5,607,550 and 5,997,690, disclose the continuous manufacture of a fibrous substrate containing about 50% to 60% SAP particles by the wet, papermaking process. In accordance with the wet process, the fibers and superabsorbent particles are mixed with copious quantities of water, or other liquid medium capable of swelling the SAP particles, and deposited onto a water-pervious support member, generally a Fourdinier wire, where a majority of the water is removed leaving a wet mass of fibers and SAP particles. The wet mat is transferred from the pervious support member and consolidated under heat and pressure to form a fibrous substrate having the SAP particles distributed throughout.
Difficult problems encountered in the continuous manufacture of an absorbent sheet material containing a relatively high percentage of SAP particles include achieving structural integrity of the sheet both during and after manufacture, and overcoming a significant loss (shakeout) of superabsorbent particles from the sheet. Another significant problem has been insufficient flexibility of the absorbent sheet, wherein a “hard,” brittle sheet difficult is to handle without breakage and impossible to form into a roll for storage, shipment, and the facile manufacture of an absorbent core.
In accordance with the present invention, it has been found, unexpectedly, that a continuous sheet material formed from unneutralized acidic water-absorbing resin and unneutralized basic water-absorbing resin particles (i.e., each 0% to about 50% neutralized), and a plasticizer, can be manufactured to provide a water-absorbent sheet material containing about 60% to 100% by weight of the acidic and basic water-absorbent resins and plasticizer. The SAPs used to form a sheet material of the present invention can be separate acidic resin particles and basic resin particles, or can be multicomponent particles containing both the acidic and basic resins in a single particle. The sheet materials have new and unexpected flexibility and structural integrity during and after manufacture, in addition to excellent fluid absorption and retention properties.